Beginning with FIG. 1A, the flavivirus genome is a single-stranded, positive-sense RNA approximately 11 kb in length, containing a 5′ untranslated region (5′ UTR); a coding region encoding the three viral structural proteins; seven nonstructural proteins, designated NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5; and a 3′ untranslated region (3′ UTR). The viral structural proteins include the capsid (C), premembrane/membrane (prM) and envelope (E) proteins. The structural and nonstructural proteins are translated as a single polyprotein. The polyprotein is then processed by cellular and viral proteases.
West Nile virus (WN) belongs to the family Flaviviridae that comprises more than 60 viruses, many of which are important human pathogens. WN is a member of the Japanese encephalitis virus (JE) serocomplex of mosquito-borne flaviviruses that includes St. Louis encephalitis, J E, and Murray Valley encephalitis viruses (Calisher, C. H. et al. 1989 J Gen Virol 70:27-43; Burke, D. S. & Monath, T. P. 2001 in: Fields Virology, eds. Knipe, D. M. & Howley, P. M. Lippincott Williams and Wilkins, Philadelphia, 4-th ed., pp, 1043-1125). Like other members of the JE antigenic complex, WN is maintained in a natural cycle that involves mosquito vectors and birds, while humans and equines are usually incidental hosts. For many years WN has been recognized as one of the most widely distributed flaviviruses with a geographic range including Africa, Australia, Europe, the Middle East and West Asia (Burke, D. S. & Monath, T. P. 2001 in: Fields Virology, eds. Knipe, D. M. & Howley, P. M. Lippincott Williams and Wilkins, Philadelphia, 4-th ed., pp. 1043-1125; Hayes, C. G. 1989 in: The Arboviruses: Epidemiology and Ecology, ed. Monath T. P. Boca Raton, Fla. CRC Press, Volume V, pp. 59-88). During 1999 WN first established itself in the USA in the Northeast and Mid-Atlantic States and more recently this virus extended its range to include the Southeastern and Western States (Anderson, J. F. et al. 1999 Science 286:2331-2333; Lanciotti, R. S. et al. 1999 Science 286:2333-2337; Campbell, G. L, et al. 2002 Lancet 2:519-529). In endemic regions, most human WN infections are asymptomatic or cause mild illness with symptoms of low-grade fever, headache, body aches, rash, myalgia, and polyarthropathy. However, human epidemics with severe disease have been reported in Israel, France, Romania, and Russia. In acute severe illness, the virus can cause hepatitis, meningitis and encephalitis leading to paralysis, and coma resulting in death. The neuropathologic lesions are similar to those of JE, with diffuse CNS inflammation and neuronal degeneration. Virus is also found in the spleen, liver, lymph nodes, and lungs of infected individuals. During the 1999 outbreak of WN in the USA, more than 60 people became ill and 7 died, while during 2002, morbidity was 3873 cases and there were 246 deaths (CDC Report: West Nile Update Current case Count, Jan. 2, 2003). Because of the recent and unexpected spread of WN from the Northeast to the Southeast and the West of the USA, this virus is considered a significant emerging disease threat that has embedded itself over a considerable region of the country. Currently, a licensed human vaccine is not available for prevention of WN disease. Mosquito control is the only practical strategy to combat the spread of disease, but effective spraying is difficult to perform in urban areas. Clearly, an effective vaccine is needed to protect at-risk populations.
Dengue viruses are mosquito-borne pathogens of the genus Flavivirus (family Flaviviridae). Four serotypes of dengue virus (DEN) have been identified, including dengue type 1 virus (DEN1), dengue type 2 virus (DEN2), dengue type 3 virus (DEN3) and dengue type 4 virus (DEN4). Live, attenuated dengue viruses of all four serotypes have been developed at Mahidol University in Thailand by passaging the wild-type viruses in primary dog kidney cell culture (Sabchareon, A. et al. 2002 Am J Trop Med Hyg 66:264-272). These are currently the least promising live, attenuated vaccine candidates for immunization against dengue virus infection and/or disease because they are not well characterized as to the relative contributions of attenuation-associated mutations to the actual mechanism of attenuation nor as to the potential for reverse mutations to revert any of the vaccine candidates to the virulent biological phenotype of the wild-type dengue virus. These vaccine candidates have been designated by a combination of their dengue serotype, the cell line through which they were passaged and the number of times they were passaged. Thus, a dengue serotype 1 wild-type virus passaged in primary dog kidney (PDK) cells 13 times is designated as DEN1 PDK13 virus. Other vaccine candidates are DEN2 PDK53, DEN3 PGMK30/FRhL3 (thirty passages in primary green monkey kidney cells, followed by three passages in fetal rhesus lung cells) and DEN4 PDK48. These four candidate vaccine viruses were derived by tissue culture passage of wild-type parental DEN1 16007, DEN2 16681, DEN3 16562 and DEN4 1036 viruses, respectively.
Except for DEN2 PDK53 virus, the number and identity of the genetic mutations that accrued during multiple passages in cell culture and that are associated with the attenuation phenotype of the vaccine candidates are unknown. Neither the relative contributions of such attenuation-associated mutations to the actual mechanism of attenuation, nor the potential for reverse mutations to revert any of the vaccine candidates to the virulent biological phenotype of the wild-type dengue virus are known for any of these four vaccine candidates. An understanding of the characteristics of a vaccine candidate is critical for the prediction of its stability and safety.
Accordingly, there is a need for attenuated, yet immunogenic flaviviruses to be used in the development of flavivirus vaccines to confer protection against flaviviruses. What would be ideal is a vaccine that would simultaneously protect an individual against flavivirus disease and be sufficiently characterized so that stability and safety are predictable.